The correct answer is (d) 40 years.
Let P be the principal amount and R be the rate of interest. We know that the simple interest earned in 20 years is equal to the principal amount, so $P = PRT$. We also know that the simple interest earned in one year is equal to $PRT/100$. Therefore, $P = PRT/100 \times 20 = 2PR/5$.
To treble the principal amount, we need to earn an additional $P$ in simple interest. This will take another $2PR/5$ years. Therefore, the total time it will take to treble the principal amount is $20 + 2PR/5 = 40$ years.
Option (a) is incorrect because it is the time it takes to double the principal amount, not treble it. Option (b) is incorrect because it is the time it takes to quadruple the principal amount. Option (c) is incorrect because it is the time it takes to quintuple the principal amount.